1. Field of the Invention
The present invention relates to an image detector and, more particularly, to a thin film transistor (TFT) type optical sensor with a conductive object or living object detection pattern.
2. Discussion of the Related Art
Generally, a thin film transistor type optical sensor comprises a window for receiving light reflected from an object, a sensor TFT for generating an optical current in proportion to the intensity of the light, a storage capacitor for storing charges of the optical current transmitted from the sensor TFT as charged information, and a switching TFT for outputting the charged information stored in the storage capacitor in accordance with an external control signal.
FIG. 1 shows a conventional TFT type optical sensor.
A sensor S1 is coupled to a controller 2 whose output end is coupled to an input end of an electrical power source 4. An output end of the electrical source 4 is electrically coupled to both the sensing part 6 and a light source 8. The sensor S1 may be replaced with a mechanical switch or an infrared switch. In operation, when a signal is output from the sensor S1, the controller 2 transmits the signal to the sensing part 6 and the light source 8 through the electrical power source 4.
FIG. 2 shows another conventional TFT type optical sensor using a mechanical switch SW1 instead of the sensor S1.
One end of the switch SW1 is coupled to an input end of the electrical power source 4 and the other end to an input end of a controller 2. An output end of the controller 2 is coupled to an input end of the electrical power source 4 whose output end in turn is coupled to both a sensing part 6 and a light source 8.
In operation, when the switch SW1 is turned “ON” to operate the optical sensor, electrical power is supplied from the electrical power source 4 to the controller 2, then input to the electrical power source 4 as a predetermined control signal. Next, the electrical power source 4 transmits electrical power to the sensing part 6 and the light source 8.
The two above-described conventional TFT type optical sensors have the following drawbacks:
1. When the switch is turned ON, since electrical power is always supplied, electrical power is unnecessarily consumed even when the sensor is not detecting an object.
2. They have no means for recognizing whether an object is living or not. For example, the sensor may be a fingerprint detection device. In that case, it is important for the sensor to discriminate between a live human finger, and, for example, a photograph or photocopy of a human fingerprint. Otherwise, the fingerprint detection device may be spoofed.